In Stefan Korcek et al., Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986, the oxidation mechanism of internal combustion engine oil is described in detail, and the literature describes the continuous production of peroxides by oxygen in the oxidation reaction. Also, S. S. Wang, Sensors and Actuators B, 17, pp. 179-185, 1994, describes the correlation between engine oil and TAN, and the lifetime of engine oil.
Regarding carbon nanotubes, Philip G., SCIENCE, VOL 287 10, 2000, reports that carbon nanotubes can be used as oxygen gas sensor materials, and J. Wang, Journal of the American Chemical Society. 125. pp. 2408, 2003, reports that carbon nanotube electrodes were used to measure hydrogen peroxide. This is because carbon nanotubes have good reactivity with oxygen. Carbon nanotubes have an sp2-bonded tubular structure of graphite, leading to a very large surface area per unit area, and thus show excellent ability to adsorb gaseous molecules or ions. Also, carbon nanotubes show the electrical properties of metals or semiconductors and have a property in which the conductance thereof changes with the adsorption of gaseous molecules or ions thereon.
Moreover, carbon nanotubes have advantages in that they have a small size and, at the same time, the ability to adsorb and store ions, show high sensitivity and response speed due to the large surface area per unit area thereof, and are excellent in physical chemical durability.
FIGS. 1a and 1b show the structure of a prior conductance-measuring sensor unit comprising carbon nanotubes. As shown in the figures, the measuring sensor unit comprises an electrode 32, to which power is applied, a conductance sensing film 31, which is applied on the electrode 32, and a substrate 33 so as to sense the electrochemical change of oil, and is formed of carbon nanotubes, and the substrate 33 on which the conductance sensing film 31 and the electrode 32 are placed. In the structure of the underlying electrode 32, there is difficulty in alignment due to opacity in an opaque silk screen process, and thus there are problems related to thickness control and smoothness.
A method for fabricating the conductance sensing unit having the above structure is as follows.
First, a step of mixing, heating and melting a binder and alpha-terpineol, that is, a step of mixing ethyl cellulose and alpha-terpineol at a given ratio, and heating and melting the mixture, is carried out.
A step of mixing carbon nanotubes with a heated and melted mixture of glass frit and alpha-terpineol, such that the carbon nanotubes can be maintained at a given viscosity, is carried out.
Then, a step of patterning the underlying electrode on the substrate using a vacuum deposition method, such as an E-beam evaporator or a sputtering method, or a screen printing method, is carried out.
Finally, a step of screen-printing the carbon nanotubes on the substrate having the underlying electrode patterned thereon, and sintering the printed carbon nanotubes, is carried out.
FIG. 2 shows the deterioration characteristics of oil, measured with a sensor fabricated according to the prior art, and FIGS. 3a and 3b are graphic diagrams showing the change in values caused by the influence of temperature in a prior oil sensor measurement method and the change in pattern of sensor output values caused by oil replacement, respectively.
As shown in FIG. 2, it can be seen that, in the values measured using the prior underlying electrode structure, the pattern of resistance measurement values definitely changes with running distance.
In the case of the conductance measuring sensor unit fabricated using carbon nanotubes through the above-described steps, when the deterioration of oil is measured, the initial sensor values differ between the sensors, and thus the ratio of the final measurement value to the initial measurement value is different for each of the sensors. Thus, in this case, in order for data values determining the degree of deterioration to be precisely measured according to the kind of oil and manufactured sensor, a very wide range of measurement data is required.
Generally, a sensor comprising a CNT sensor film is fabricated to have an initial value of about 0.5-20 kΩ. Conductance or electrical resistance, actually measured in oil, changes according to the condition of oil. Sensors having a low initial value have a small change in measurement values, and sensors having a relatively large change in measurement values have a somewhat large change in measurement values.
In the prior art, a conventional sensing method adopts a method of analyzing a change in oil condition (using absolute values) and comparing the analyzed values to reference data. If initial values are different between sensor products, the reference database will require a very vast amount of data according to a fabricated sensor, and thus a large amount of time and effort will be required to make the database.
In the prior art, due to the above-described problems, a method of sensing the condition of oil using an oil sensor comprising a CNT sensor film is used, and a method of sensing the change in condition of oil by recognizing the slope pattern of measurement values of a sensor is also used. However, in this case, the oil sensor having the CNT sensor film is greatly influenced by temperature, and thus recognizing the slope pattern involves significant errors. Also, once the oil sensor having the CNT sense film is used, it cannot be returned to the initial value state of oil, when oil is replaced with fresh oil. Thus, the data that is first measured and the data that is subsequently measured are different from each other, and in this case, the slope patterns of the measurement values are different, leading to significant errors.
Meanwhile, a prior method for measuring total acid number has problems in that a measurement apparatus is difficult to fabricate due to its complex structure and large size, and is sensitive to noises, leading to erroneous output.
In addition, in the prior art, there is a problem in that real-time measurement is difficult, because the measurement of total acid number and other properties of oil is performed using a measurement sensor in an operation-stopped state or a state in which the operation of a transformer is stopped.